Heretofore, complex-shaped parts made from metal powders were completely formed by an appropriate forming process such as, for example, investment casting or machining. An alternative method is the metal injection molding process. Metal-injection-molding (MIM) is recognized as a premier forming method for complex shapes. It affords significant advantages over other forming methods, by being capable of rapidly producing net shape, complex parts in high volume. Initially, MIM comprised the step of mixing metal powder with a dispersant and a thermoplastic organic binder of variable composition. The molten powder/binder mixture was heated during the injection molding process and injected into a relatively cold mold. After solidification, the part was ejected in a manner similar to plastic parts. Subsequently, the binder was removed and the part was densified by a high temperature heat treatment. There were a number of critical stages in this process, which included the initial mixing of the powder and binder, the injection of the mixture into the mold, and the removal of the organic matrix material. One of the main disadvantages of the inital MIM process is the removal of the organic vehicle. At present, with the MIM process the cross section limit for fine particle sizes is 0.5-0.75 inch (12.7-19.05 mm). If the particle sizes exceed that limit, the binder removal process will lead to defects, pinholes, cracks, blisters etc. Binder removal takes place by slow heat treatment processes that can take up to several weeks. During debinding at elevated temperatures, the binder becomes a liquid, which can result in distortion of the green part due to capillary forces. Another disadvantage of the initial MIM process is the tendency for the relatively high molecular weight organic material to decompose throughout the green body, causing internal or external defects. The use of solvent extraction, wherein a part of the organic material is removed using an organic or supercritical liquid, sometimes minimizes defect formation. Solvent extraction encounters difficulties because the remainder still needs to be removed at elevated temperatures. However, the solvent exaction process allows for the formation of porosity throughout the part, with the result that removal of the remaining organic is facilitated. During binder removal, part slumping can pose problem, especially for the larger particle sizes if the green density/strength is not high enough.
As such, MIM offers certain advantages for high volume automation of net shape, high dimensional control and complex parts, but the limitation of part size and the very long binder removal times combined with their environmental impact has not resulted in the expected growth of the use of this technique.
Some improvements, such as the use of water based binder systems, have been made to the initial PIM process. Hens et al. developed a water leachable binder system [U.S. Pat. No. 5,332,5373]. The injection molding feedstock is made with a tailored particle size distribution (to control the rheology); a PVA based majority binder, and a coating on each of tee binder particles. During molding, these coatings form necks that give the part rigidity. After injection molding, there is a water debind that lasts several hours. After the remaining binder is cross-linked by either UV or chemical methods, the part undergoes a thermal debind, which takes 8-12 hours for a part such as a golf club head. Other aqueous-based binders contain either polyethylene glycols, PVA copolymers, or COOH-containing polymers. A polyacetal-based system also has been developed that is molded at moderately high temperatures, after which the binder is removed by a heat treatment with gaseous formic or nitric acid. The low temperature excludes the formation of a liquid phase and thus distortion of the green part due to viscous flow. The gaseous catalyst does not penetrate the polymer and the decomposition only takes place at the interface of the gas and binder, thereby preventing the formation of internal defects. These improvements are limited by the requirement for separate binder removal furnaces and relatively long times, depending on the part size. There are environmental issues as well with removal of the large amount of wax/polymer in the form of fire hazards and volatile organic compound discharge.
Honeywell International Inc. has developed an injection molding process using agar as an aqueous binder (e.g., U.S. Pat. No. 4,734,237 to Fanelli et al., the disclosure of which is expressly incorporated herein by reference). In this patent, this binder system is applied to both ceramic and metal powders. The use of agarose or derivatives of polysaccharide aqueous gels are also included. The advantage over state-of-the-art wax-based technology is the use of water as the fluid medium versus wax. In such feedstocks, water serves the role of the fluid medium in the aqueous injection molding process, comprising roughly 50 vol % of the composition, and agar provides the "setting" function for the molded part. The agar sets up a gel network with open channels in the part, allowing easy removal of the water by evaporation. The agar is eventually removed thermally; however, it comprises less than 5 volume fraction of the total formation.
The lack of a debind requirement makes these parts particularly suitable for co-sintering. In the co-sintering process, a component which has multiple portions that may vary in dimensions and shape can be more easily fabricated by joining two easily designed parts during sintering rather than completely molding one more complicated design. Instead of designing a separate mold for each different dimensional variation and shape, and then manufacturing each part separately, it has been found to be advantageous to manufacture distinct portions of a complex part separately and fuse or bond the portions together to form the final desired part. This also allows part production flexibility, since a base portion can be molded and stored green in large quantities while several adjunct parts of different designs can be molded on an as-needed basis. The work-in-progress can be reduced, facilitating just-in-time delivery.
Among the problems associated with achieving this result have been the quality of the bond or the overall strength of the joint. Approaching the strength of an integrally formed molded part has proven to be difficult to achieve. Accordingly, it would be desirable to provide a method for joining similar materials which avoided the above-mentioned problems.